CueSettingSize.NoKeyword -- Incorrect assertions

We are asserting a `WEBVTT_INVALID_CUESETTING`, which is technically a reasonable error to return, however given that we have a `WEBVTT_MISSING_CUESETTING_KEYWORD` error available, it is currently thrown if we encounter `:` before a keyword, in lieu of `WEBVTT_INVALID_CUESETTING`.

So there are two ways to go about this, either the test changes, or the code changes. Do we need to throw `WEBVTT_MISSING_CUESETTING_KEYWORD`? Or is `WEBVTT_INVALID_CUESETTING` sensible enough?